Wide-angle, high-speed, free-space optical communications system

ABSTRACT

A free-space optical communications system for transmitting data between an aircraft computer system (14) and a ground-based computer system (12). The system includes a pair of corresponding optical transmitters (36) and optical receivers (38) that transmit and receive optical signals transmitted between the two computer systems. Included within each optical transmitter is one or more light-emitting diodes (42) that produce optical signals corresponding to the data to be transmitted. A beam-forming horn (44) is bonded directly to the light-emitting diodes to direct the optical signal uniformly over a target area. The optical receiver includes one or more infrared windows (50) to reduce the mount of ambient light received by the optical receiver. A compound parabolic concentrator (64) collects light transmitted from the optical transmitter and directs the light onto an avalanche photodiode (66), which includes thermal bias compensation. An AC network couples the output signal of the photodiode to a transimpedance amplifier (70). An optional optical shroud (34) surrounds the optical transmitters and receivers to further reduce the amount of ambient light that is received by the optical receivers.

RELATED APPLICATION

The present application is a continuing application of our prior U.S.patent application Ser. No. 07/943,328 filed Sep. 10, 1992, now U.S.Pat. No. 5,319,446, titled "Wide-Angle, High-Speed, Free-Space OpticalCommunication System" which is expressly incorporated by referenceherein.

FIELD OF THE INVENTION

The present invention relates to communications systems in general and,in particular, to infrared, free-space optical communications systems.

BACKGROUND OF THE INVENTION

In the last 20 years, computers have played an ever increasing role inthe airline industry. For example, computers are used onboard anaircraft for such tasks as aiding navigation, scheduling maintenance,monitoring the operation of equipment as well as for controlling theposition of the flight control surfaces to fly the aircraft. On theground, computers are used to ticket passengers, keep track of luggage,maintain records of seat availability, schedule departure changes, etc.In the past, there has only been a limited exchange of data between theaircraft computer system and the ground-based computer system used by anairline. Such exchange usually took place by hand carrying a floppy diskbetween the two computer systems.

Since the invention of computer networks, there has been considerableeffort within the airline industry devoted to developing a communicationsystem that connects the aircraft computer system to the ground-basedcomputer system. Early efforts used digital radio but were unsuccessfulbecause of the limited bandwidth available due to radio frequencyspectrum allocation and contention/interference. One suggested method ofestablishing a higher bandwidth communication system was to connect thetwo computer systems together using a fiber optic communications link.In such a system, a fiber optic cable would extend from the ground-basedcomputer system to a fiber optic cable connector disposed on the side ofthe aircraft. As the aircraft taxied into a dock, a member of themaintenance crew could plug the fiber optic cable into the side of theaircraft, thereby allowing the data communication to take place.However, such a solution was deemed undesirable due to the fragilenature of fiber optic cable connectors and the need for ground crewaction. Additionally, it is possible that the aircraft could pull awayfrom the dock without disconnecting the cable, causing subsequent delaysand extensive damage to the fiber optic cable and aircraft.

To overcome the problems associated with a fiber optic cable-basedcommunication system, an alternate communications scheme was suggestedby the airlines industry. The alternate scheme involved the use of afree-space optical communications system that could transmit informationbetween the aircraft computer system and the ground-based computersystem using a modulated infrared light beam. The free-space opticalcommunications system would eliminate the need for the fiber optic cablepossible damage from the aircraft pulling away and disconnecting thecable. However, current free-space optical communications systems sufferfrom at least three problems that, in combination, prevent suchcommunications systems from being readily usable in an aircraft toground-based computer communication link. First, current free-spaceoptical communications systems do not operate at the high data rate thatthe airlines are requiring for a commercially viable communicationsystem. For example, the Aeronautical Radio Incorporated (ARINC)standards group is currently developing a communications protocol thatrequires data communication between an aircraft and a ground-basedcomputer system be accomplished at speeds of 100 Mbits/sec. Second,current state of the art high-speed, free-space optical communicationssystems often have a narrow field of view and, as such, requireadditional control systems to align the optical transceivers to ensureproper data transmission. Including such control systems into afree-space optical communications system adds significantly to the costof the system, as well as introduces a likely source of system failure.Finally, current free-space optical communications systems will notoperate in all types of weather conditions experienced at an airport.

Therefore, a need exists for a free-space optical communications systemthat can transmit data between an aircraft and a ground-based system athigh speeds over all weather conditions. Additionally, the communicationsystem should have a wide field of view to eliminate the need for anycontrol systems to align the optical components of the system.

SUMMARY OF THE INVENTION

The present invention comprises a free-space optical communicationssystem that transmits data between two computer systems at high speed,in all weather conditions, and without the need for precise alignmentmechanisms. In the preferred embodiment of the present invention, thecommunication system is used to transmit information between an aircraftcomputer system and a ground-based computer system. The system includesa pair of optical transceivers, one of which is located on the aircraftand the other preferably located on an adjacent passenger loadingbridge. Each transceiver includes an optical transmitter having one ormore light-emitting diodes (LEDs) that produce optical signalscorresponding to the data to be transmitted. Optically coupled to theLEDs is a nonimaging optical device such as a beam-forming horn to focusand uniformly distribute the optical signals over a target area in whichthe optical signals are to be received. Each transceiver also includesan optical receiver having one or more optical filters to reduce themount of ambient light entering the receiver. A nonimaging opticalcollector such as a compound parabolic concentrator (CPC) is coupledwith the optical filters and collects a portion of the optical signalsproduced by the optical transmitter. The CPC is optically coupled to aphotodiode, such as an avalanche photodiode (APD), which produces anelectrical output signal that corresponds to the optical signalsreceived. The APD diode includes a biasing voltage supply havingtemperature compensation and automatic gain control (AGC) to allowoperation over a wide temperature and signal level range. A currentshunt is connected to the output of the APD diode to shunt away aportion of the output signal that is produced due to any ambient lightcollected by the CPC. An AC coupling means extracts a time varyingportion of the output signal and feeds the time varying portion to anamplifier. The output of the amplifier is coupled to the receivingcomputer system. An optional optical shroud extends between the adjacentpassenger loading bridge and the aircraft to surround the pair ofoptical transceivers and reduce the amount of ambient light that iscollected by the optical receivers.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a free-space optical communications system according to thepresent invention that transmits data between an aircraft computersystem and a ground-based computer system;

FIG. 2 shows an optional optical shroud that extends from a passengerloading bridge to the aircraft;

FIG. 3 shows a block diagram of the free-space optical communicationssystem according to the present invention;

FIG. 4 shows an optical transmitter and an optical receiver according tothe present invention;

FIG. 5 shows an optical window located on the exterior of the aircraftfor transmitting and receiving optical communication signals; and

FIG. 6 shows an alternative arrangement of the transmitter and receiverlocated on the exterior of the aircraft.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a duplex, free-space optical communicationssystem for transmitting information between two computer systems. Asdiscussed above, the preferred embodiment of the present invention isused to transmit information between an aircraft computer system and aground-based computer system. However, the present system could also beused to transmit information between any two computer systems,stationary or mobile, such as an automobile computer and a toll boothcomputer system, or from a computer system on a train to a ground-basedcomputer system, etc.

FIG. 1 shows an aircraft 10 parked near a passenger loading bridge 20.As will be further described below, the free-space opticalcommunications system 30 according to the present invention allows datato be transmitted using infrared light beams that are transmittedbetween an optical transceiver located behind an infrared windowdisposed in the side of the aircraft and a corresponding opticaltransceiver located underneath a passenger loading bridge 20. Thetransceiver disposed on the underside of the passenger loading bridge 20is coupled to the ground-based computer system via a communicationscable 40 such as a fiber optic cable. An optional optical shroud 34 maybe mounted on the underside of the passenger loading bridge 20. Thefree-space optical communications system can transmit data between theaircraft computer system (not shown) and the ground-based computersystem (also not shown) at a rate of 100 Mbits/sec. Additionally, aswill be further described below, the free-space optical communicationssystem according to the present invention has wide transmission beamsand corresponding wide fields of view to compensate for misalignmentsbetween the aircraft 10 and the passenger loading bridge 20, and needsno active control mechanisms to align the optical transceivers.

FIG. 2 shows how the free-space optical communications system accordingto the present invention is disposed underneath the passenger loadingbridge 20 to transmit information between the aircraft computer systemand the ground-based computer system. If the optical communicationssystem is to be used in conditions of bright sunlight, the opticalshroud can be used to reduce the amount of ambient light that enters thetransceivers. The optional optical shroud 34 extends between thepassenger loading bridge 20 and the aircraft 10 to reduce the amount ofambient light that reaches the optical transceivers when the aircraft isparked at the passenger loading bridge. The optical shroud 34 has ahorn-shaped construction with a cross-sectional area slightly largerthan the transmitted beam from the transceiver disposed on the passengerloading bridge 20.

The communications cable 40 extends between the optical transceiverlocated under the passenger bridge 20 and the ground-based computersystem (not shown) to carry the data to be transmitted by and receivedfrom the ground-based computer system.

As will be further discussed below, the optical transmitters transmitinfrared optical signals uniformly over large target areas.Correspondingly, the optical receivers have large fields of view. Thisensures that data communication can take place regardless of where inthe target area a corresponding transceiver is located. In the preferredembodiment, the aircraft transceiver includes an infrared window 50 thatcovers the transceiver and is located flush with the surface of theaircraft 10 in a position that is generally below a door 17. When theaircraft is properly docked next to the passenger loading bridge 20, theinfrared window 50 will be located within the respective target area.

A block diagram of the free-space optical communications systemaccording to the present invention is shown in FIG. 3. The free-spaceoptical communications system 30 transmits data bidirectionally betweena ground-based computer system 12 and an aircraft computer system 14.The system includes a pair of optical transceivers each of whichincludes a separate optical transmitter 36 and an optical receiver 38.To reduce the amount of ambient light that enters the optical receivers38, the transceivers include a pair of infrared windows 50. Data istransmitted between the two computer systems 12 and 14 via a modulatedinfrared light beam produced by the optical transmitters 36. Eachoptical transceiver is coupled to its respective computer system via acommunications cable 40. In the preferred embodiment of the presentinvention, data is transmitted in full duplex between the ground-basedcomputer system 12 and the aircraft computer system 14.

A more detailed view of the optical transmitter 36 and the opticalreceiver 38 is shown in FIG. 4. For purposes of illustration, FIG. 4shows one optical transmitter 36 and a corresponding optical receiver38. However, as will be appreciated, the system includes a secondoptical transmitter and corresponding optical receiver to transmit datain the opposite direction. The optical transmitter 36 includes one ormore infrared light emitting diodes (LEDs) 42 driven by a transistor 43.An electronic signal that corresponds to the data to be transmitted isapplied to the base electrode of the transistor 43, causing thetransistor 43 to conduct current and in turn causing the one or moreLEDs 42 to produce an infrared optical signal. In the presentlypreferred embodiment of the invention, the LEDs 42 comprise a singlehigh power infrared LED to produce the infrared optical signal. However,if more optical power is required, a plurality of LEDs connected eitherserially or in parallel could be used.

Because the infrared light produced by the one or more LED 42 extends atwide angles from the face of the LED, a beam-forming horn 44 is placedaround the LED 42. As a result, the beam-forming horn 44 collects theinfrared light and directs it towards the corresponding optical receiver38. Preferably, the beam forming horn is a hollow, metal device made ofgold-plated nickel to provide the required reflectivity. Alternatively,the beam-forming horn can be made of any type of transparent opticalmaterial, including plastic or glass. The dimensions of the beam-forminghorn are chosen depending on the size of the target area and theseparation between the transceivers. The beam-forming horn should takeinto account the illumination pattern of the LED and if the horn is madeof a transparent material, the difference in the index of refractionbetween the transparent material and the surrounding air so thatinfrared light produced by the LED 42 is directed with equal intensityover the entire target area. The angles of the sides of the horn areapproximately equal to the angles of the optical beam required to coverthe target area. The design parameters are defined to optimize opticalintensity in the target area.

If the LED has sufficient optical power, the beam-forming horn may bereplaced with a traditional imaging lens or a compound parabolicconcentrator bonded directly to the LED with an optical grade epoxy.However, the inventors have found that the use of a beam-forming hornprovides the best optical communication link under most conditions.

The beam-forming horn 44 increases the optical power that is transmittedin the direction of the receiver as well as distributes the opticalsignals evenly over the target area. As described above, the target areais a rectangular section. The size of the target area compensates forvariations or misalignments between the aircraft and the passengerloading bridge. If the aircraft is parked such that the correspondingoptical receiver 38 is located anywhere in the target area, thencommunication can take place between the aircraft and ground-basedcomputers.

Disposed at the output end of the beam-forming horn 44 is an infraredwindow 50a. The infrared window 50a passes light having frequencies inthe infrared range and serves to protect the optical transmitter fromdirt, rain, etc.

The optical receiver 38 includes an infrared window 50b that preferablyonly passes light having frequencies in the infrared range of theoptical transmitter 36. Infrared light traveling through the infraredwindow 50b is collected by a dielectric, compound parabolic concentrator(CPC) 64. The CPC 64 is optically coupled to the light-sensitive surfaceof a photodiode 66. Preferably, the photodiode 66 is an avalanche photodetector (APD) type that conducts an electrical current that isproportional to the level of light received. However, other types ofphoto detectors could be used, such as a PIN photodiode. The CPC 64 hassuperior light-gathering properties as compared to the imaging lenstypically used with optical detectors. The CPC has a large far field ofview and can receive light from anywhere in the target area with highefficiency. The design parameters of the CPC are chosen to maximize theoptical power detected in the target area. This large field of view isalso sharply defined such that any light outside the field of view isnot directed to the APD diode 66. This property also helps reduce theamount of ambient light received by the diode. Finally, thelight-gathering properties of the CPC are nearly uniform across itsfield of view. Therefore, there are no "dead spots" within the CPC'sfield of view that would attenuate any optical signal detected. The CPC64 is preferably made of a dielectric material bonded directly to thelight-gathering surface of the APD diode 66 with an optical grade epoxy.The details of how to construct a compound parabolic concentrator arewell known to those skilled in the optical arts and therefore will notbe discussed further.

In some cases it may be desirable to place a cylindrical lens directlybehind the infrared window 50b and in front of the CPC 64. When used,the cylindrical lens serves to modify a circular field of view of a CPC64 into an elliptical field of view, where a noncircular field of viewis needed. If used, the cylindrical lens increases the light that isdetected by receiver 38 in one axis.

The APD diode 66 is biased with a temperature compensated high voltagesource 68 that produces photo current gain that does not substantiallyvary over wide temperature ranges. Disposed between the high voltagesource 68 and the APD diode 66 is a resistor R₁ and a capacitor C₁. Theresistor R₁ acts to provide automatic gain control for the opticalreceiver 38 as follows. In general, the electrical current that isgenerated by an APD diode 66 for a given mount of light increasesexponentially as the bias voltage increases. However, as the APD diode66 in the receiver 38 conducts more and more current due to more lightbeing received, the voltage drop across the resistor R₁ increases,thereby reducing the voltage that biases the APD diode 66, causing theAPD diode to conduct less current. This negative feedback action tendsto maintain the level of current conducted by the APD diode relativelyconstant despite fluctuations in the mount of input light receivedthereby preventing the APD diode from saturating, and keeping the outputsignal produced by the APD diode relatively uniform in magnitude. Thecapacitor C₁ operates in conjunction with the resistor R₁ such that thenegative feedback is determined by slow or average variations of thereceived optical signal strength.

The temperature compensation provided by the high voltage source 68 isaccomplished by comparing a fixed fraction of the bias voltage appliedto the APD diode to a reference voltage produced by an IC temperaturesensor (not shown) within the high voltage source 68. The differencebetween these two voltages drives a high-gain, negative feedback circuitthat includes a fixed gain DC to DC converter (also not shown) so thatthe bias voltage adjusts over temperature to maintain constant APDcurrent gain.

Connected between the output of the APD diode 66 and ground, or otherreference potential, is a resistor R₂. Although the infrared window 50bremoves most of the ambient light that may be impinging upon the APDdiode 66, it is invariable that some ambient light will be collected bythe CPC 64 and be passed to the APD diode. A coupling capacitor C₂ isconnected between the output of APD diode 66 and a transimpedanceamplifier 70. The capacitor C₂ passes only a time-varying portion of theoutput signal to the amplifier whereas current conducted by the diode 66due to the ambient light is shunted to ground by resistor R₂. Thetime-varying portion of the output signal is directly proportional tothe power of the optical signals produced by the optical transmitter 36and received by the APD diode 66.

The transimpedance amplifier 70 with appropriate additional circuitry(not shown) converts an AC current flowing through the capacitor C₂ intoa digital voltage signal that is transmitted to either the aircraftcomputer system 14 or the groundbased computer system, depending onwhether the optical receiver 38 is located on the aircraft or on theground.

As stated above, the free-space communications system according to thepresent invention also includes another optical transmitter and anotheroptical receiver that transmit and receive data in the reversedirection. Together, these pairs of corresponding optical transmittersand receivers allow full duplex communication to take place between theaircraft computer system and the ground-based computer system. Inoperation, it is not necessary to vary the frequency of the opticalsignals that are transmitted from the aircraft to the ground-basedcomputer system with respect to the frequency of the optical signalsthat are transmitted from the groundbased computer system to theaircraft. Full duplex transmission can take place if precautions aretaken to ensure that the optical signals transmitted by a transceiver'soptical transmitter are not received by the transceiver's own opticalreceiver. This may be accomplished in the present invention by splittingeach transceiver's window into two halves 50a and 50b as shown anddiscussed in further detail below.

FIG. 5 shows an infrared window disposed on the exterior surface of anaircraft 10. The window is divided into two separate infrared windows50a and 50b similar to those shown in FIG. 4. The window is mountedflush with the outer surface of the aircraft 10. A divider 55 separatesthe two infrared windows 50a and 50b to prevent light directed by thebeam-forming horn 44 from leaking into the CPC 64. By preventing suchleakage or cross-talk, the communication system according to the presentinvention can operate in full duplex at high data rates using the samefrequency light pulses for data transmission in both directions.

An alternative and currently preferred arrangement for mounting theinfrared transmitter and receiver on the exterior of the aircraft isshown in FIG. 6. In this arrangement, a pair of round infrared windows72 and 70 are mounted in a single plate 74, which in turn is mountedflush with the exterior skin of the aircraft 10 using a plurality ofrivets 75 or other suitable fastening mechanisms. Behind the window 73is the beam-forming horn 44 for use in transmitting the infrared opticalsignal. Behind the window 72 is the CPC 64 used to receive the infraredoptical signals. The physical separation of the infrared windows 72 and73 reduce the possibility that the signal transmitted from atransceiver's transmitter section will be received by the transceiver'sown receiver.

As will be appreciated, the communication system according to thepresent invention is "passive" in the sense that no special equipment isneeded to align the optical transceivers. This has the benefit of notonly being cheaper to manufacture but is also less likely to malfunctionas the communication system is exposed to the environment.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention. Itis therefore intended that the scope of the invention be determinedsolely from the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A free-space opticalcommunications system for transmitting data from an aircraft computersystem to a ground-based computer system, comprising:an opticaltransmitter coupled to the aircraft computer system for producingoptical signals that correspond to the data to be transmitted betweenthe aircraft computer system and the ground-based computer system,wherein the optical transmitter includes:(a) a light source thatproduces optical signals corresponding to the data to be transmitted;(b) means for gathering the optical signals produced by the light sourceand for distributing the optical signals uniformly over a target area;an optical receiver coupled to the ground-based computer system, forreceiving the optical signals produced by the optical transmitter,wherein the optical receiver includes:(a) optoelectronic means forproducing an output signal that corresponds to the data transmitted; and(b) a compound parabolic concentrator coupled to the optoelectronicmeans, for gathering a portion of the optical signals produced by thelight source and for directing the portion of the optical signalsgathered to the optoelectronic means.
 2. The free-space opticalcommunications system of claim 1, further comprising:an optical shroudthat extends between the optical transmitter and the optical receiverfor reducing the amount of ambient light that is received by the opticalreceiver coupled to ground-based computer system; and light-filteringmeans disposed in front of the optoelectronic means for reducing themount of ambient light directed to the optoelectronic means.
 3. Thefree-space optical communications system of claim 1, wherein the opticalreceiver further includes means for removing a portion of the outputsignal due to ambient light received by the optical receiver.
 4. Thefree-space optical communications system of claim 1, furthercomprising:gain control means coupled to the optoelectronic means, forregulating the magnitude of the output signal produced by theoptoelectronic means.
 5. The free-space optical communications system ofclaim 1, further including:a temperature-compensated biasing means forproviding a bias voltage to the optoelectronic means.
 6. The free-spaceoptical communications system of claim 1, wherein the means forgathering the optical signals produced by the light source and fordistributing the optical signals uniformly over a target area comprisesa hollow horn having a reflective interior coating.
 7. The free-spaceoptical communications system of claim 1, wherein the means forgathering the optical signals produced by the light source and fordistributing the optical signals uniformly over a target area comprisesa solid horn-shaped prism made of a substantially transparent material.8. The free space optical communications system of claim 7 wherein themeans for gathering the optical signals produced by the light source andfor distributing the optical signals uniformly over a target areacomprises a compound parabolic concentrator.
 9. The free-space opticalcommunications system of claim 1, wherein the means for gathering theoptical signals produced by the light source and for distributing theoptical signals uniformly over a target area comprises a lens disposedin front of the light source.
 10. A free-space optical communicationssystem for transmitting data bidirectionally between an aircraftcomputer system and a ground-based computer system, comprising:a firstoptical transmitter coupled to the aircraft computer system and a secondoptical transmitter coupled to the ground-based computer system, eachtransmitter including:(a) a light source that produces optical signalscorresponding to the data to be transmitted between the aircraftcomputer system and the ground-based computer system; (b) means forcollecting the optical signals produced by the light source anddistributing the optical signals uniformly over a target area; and afirst optical receiver coupled to the ground-based computer system and asecond optical receiver coupled to the aircraft computer system thatreceive the optical signals transmitted from a corresponding opticaltransmitter, wherein each optical receiver includes:(a) a photodiodethat produces an output signal that is proportional to a receivedoptical signal; (b) a compound parabolic concentrator bonded to thephotodiode that collects the optical signals transmitted from theoptical transmitter and directs the optical signals onto the photodiode;(c) current shunt means coupled to an output of the photodiode forshunting away a portion of the output signal that is due to ambientlight collected by the compound parabolic concentrator.
 11. Thefree-space optical communications system of claim 10, wherein the firstand second optical receiver each includes:an optical filter disposedbetween the corresponding optical transmitter and the photodiode. 12.The free-space optical communications system of claim 10, wherein thefirst and second optical receivers each include:atemperature-compensated voltage supply that provides atemperature-compensated biasing voltage to the photodiode.
 13. Thefree-space optical communications system of claim 10, wherein the firstand second optical receivers each include:means for extracting a timevarying portion of the output signal produced by the photodiode.
 14. Afree-space optical communications system for transmitting databidirectionally between an aircraft computer system and a ground-basedcomputer system, comprising:a pair of optical transmitters, one of whichis coupled to the aircraft computer system and another of which iscoupled to the ground-based computer system, wherein each of the opticaltransmitters includes:(a) one or more light-emitting diodes (LEDs) thatproduce optical signals that correspond to data to be transmitted; (b) ahollow horn having a reflective interior surface that surrounds the oneor more LEDs for uniformly distributing optical signals produced by theLEDs over a target area, and a pair of optical receivers, one of whichis coupled to the aircraft computer system and another of which iscoupled to the ground-based computer system, wherein each opticalreceiver includes:(a) means for gathering light that impinges upon theoptical receiver; (b) a photodiode bonded to the means for gatheringlight, the photodiode producing an electrical signal that isproportional to the amount of light gathered; and (c) filter meansdisposed in front of the means for gathering light for reducing theamount of ambient light received by the photodiode.
 15. The free space,optical communications system of claim 14, wherein the means forgathering light comprises a compound parabolic concentrator.
 16. Amethod of optically transmitting data between an aircraft computersystem and a ground-based computer system, comprising the stepsof:producing an infrared optical signal that corresponds to the data tobe transmitted; directing the infrared optical signal through lightdistributing means so that the optical signal is evenly distributedwithin a target area in which an optical receiver is positioned;gathering light at the optical receiver using a compound parabolicconcentrator that is bonded to a photodiode, the photodiode producing anelectrical signal proportional to the amount of light gathered; removinga component of the electrical signal produced by the photodiode due toambient light gathered by the compound parabolic concentrator; andcoupling an AC component of the electrical signal produced by thephotodiode to an amplifier, wherein a magnitude of the AC component ofthe electrical signal is proportional to the data transmitted betweenthe aircraft computer system and the ground-based computer system. 17.An optical transceiver adapted to be coupled to an exterior of anaircraft for transmitting and receiving data between an aircraftcomputer system and a ground-based computer system comprising:an opticaltransmitter for producing optical signals that correspond to data to betransmitted from the aircraft computer system to the ground-basedcomputer system, including:(a) one or more light sources coupled to theaircraft computer system for producing the optical signals correspondingto data that is transmitted from the aircraft computer system to theground-based computer system; (b) light distributing means for gatheringthe optical signals and distributing the optical signals uniformly overa target area; an optical receiver for receiving optical signals thatare transmitted from a second optical transmitter coupled to theground-based computer system, including:(a) a photodiode that producesan output signal that is proportional to a received optical signal; (b)a compound parabolic concentrator bonded to the photodiode that collectsoptical signals and directs the collected optical signals onto thephotodiode; and (c) a current shunt coupled to the output signalproduced by the photodiode that shunts a portion of the output signaldue to ambient light collected by the light-collecting means.
 18. Anoptical communications system for transmitting data bidirectionallybetween a first computer system and a second computer system,comprising:an optical transmitter coupled to the first computer systemfor producing optical signals that correspond to the data to betransmitted between the first computer system and the second computersystem, wherein the optical transmitter includes:(a) light-producingmeans for producing optical signals corresponding to the data to betransmitted; (b) light distributing means for gathering the opticalsignals produced by the light-producing means and for distributing theoptical signals uniformly over a target area; an optical receivercoupled to the second computer system for receiving the optical signalsproduced by the optical transmitter, wherein the optical receiverincludes:(a) a photodiode for producing an output signal that isproportional to a received optical signal, the output signal of thephotodiode being coupled to the second computer system for producing anoutput signal that corresponds to the data transmitted; (b) a compoundparabolic concentrator bonded to the photodiode for gathering a portionof the optical signals produced by the light-producing means and fordirecting the portion of the optical signals gathered to the photodiode;and (c) light-filtering means disposed in front of the compoundparabolic concentrator for reducing the amount of ambient light receivedby the photodiode.
 19. The optical communications system of claim 18,wherein the optical receiver further includes current shunt meanscoupled to the photodiode for removing a portion of the output signaldue to ambient light received by the optical receiver.
 20. The opticalcommunications system of claim 18, further comprising:gain control meanscoupled to the photodiode for regulating the magnitude of the outputsignal produced by the photodiode.
 21. The optical communications systemof claim 18, further including:a temperature-compensated biasing meansfor providing a bias voltage to the photodiode.